A thermocolor ribbon, also referred to as a thermal-tranfer ribbon is a ribbon which is capable of use in a thermal-transfer process in which the ribbon is provided with a color-transfer ribbon, i.e. the melt color can be transferred upon being raised to a melting temperature of this layer, portionwise and in a pattern as noted, to a substrate which receives the print.
The substrate can be, as noted, a paper sheet which can be displaced by a platen relative to the print head and the print head can be provided to engage the ribbon on the side of the carrier opposite that which is provided with the melt color to press the ribbon against the substrate at the requisite temperature to effect a melting of the color-transfer layer at least in a local region at which such pressure is applied and to effect the transfer of an appropriate symbol selected in the head to the substrate.
Generally the color-transfer layer comprises a wax-bonded or plastic-bonded melt color, at least on the side turned toward the substrate which receives the print and on which the transfer portion of the melt color is bonded to form the printed symbol thereon.
Thermocolor ribbons, also known as thermocarbon ribbons when the pigment of the color-transfer layer includes or consists of carbon black, have long been known. Generally they comprise a foil-like carrier, for example of paper, a plastic or the like, and a color-transfer layer in the form of the melt color applied thereto. The color-transfer layer is in the form of a plastic-bonded and/or wax-bonded coloring agent or carbon black layer.
The melt color of the thermocolor ribbon can be melted by the action of a thermal printing head for transfer to a receiving substrate which can be a printing paper or some other suitable paper substrate. Thermal printers or thermal printer heads which can be used for this process are known, for example, from German printed applications DE-AS 2,062,494 and 2,406,613, as well as from German open application DE-OS 3,224,445. Such ribbons as may be used with these printers can be referred to as "TCR" ribbons, i.e. thermal-carbon ribbons. The thermal printing head of the printer can generate the symbols, such as alphanumeric characters, as heated points which press against the reverse side of the ribbon, i.e. the side opposite the side provided with the melt color. The heated symbol, such as an alphanumeric character of the printing head, can be at a temperature of about 400.degree. C. which is sufficient to locally melt the melt color at the heated selection and effect transfer of the locally heated pattern as it comes into contact with the paper sheet. The used portion of the thermal-color ribbon can be taken up on a spool.
The thermocolor ribbon can be provided with a plurality of different melt colors adjacent one another. For example, with a combination of the basic colors blue, yellow and red, it is possible to produce colored printed images. The advantage of this process, by comparison to conventional color photography, is that the disadvantageous development and fixing steps can be eliminated. Thermal printers can operate with great printing speeds. For example, a German Industrial Standard DIN A4 page can be printed in 10 seconds without detrimental noise generation.
Mention may also be made of another process which also uses thermocolor ribbons but wherein the symbol transfer is not effected by the use of a heated symbol of the printing head, but rather is a consequence of resistance heating generated in a special foil-like character.
The melt color here forms a functional layer which is locally brought to a temperature sufficient to melt the layer by the resistance heating applied for transfer of the symbol.
Since the ribbon is electrically conductive, in the field the process is referred to as an electrothermal process and the ribbon has an electrothermal ribbon (ETR). A corresponding thermal-transfer printing system is described, for example, in U.S. Pat. No. 4,309,117.
It has been found that in thermal-printing processes of the aforedescribed type, sometimes difficulties arise in the release of the melt color from the heated location of the carrier material.
To solve this problem a variety of proposals can be found in the art. For example, in European patent application 86,301,743 (publication No. 0 194 860) for example, it is proposed to provide between a plastic-bonded melt color and its carrier or support, a thermally activated meltable "release layer" which contains as its principal component, preferably, a wax.
The disadvantage of this system is that it cannot be used with advantage for those thermocolor ribbons which utilize a wax-bonded melt color.